This invention relates generally to electronic appliance control systems and, more particularly, to a full half cycle triac control system for glass cook top applications.
Glass cook top units generally employ either halogen or radiant heating elements to heat items placed thereon and to provide a radiant glow which is visually indicative of the current power level. Generally, electronic or electro-mechanical systems used to control the transmission of energy from a supply source to the heating elements are turned fully ON when a 100% power level is selected but are pulsed to provide less than full power. This method typically enables efficient and effective heating element control. However, providing a uniform heating element glow or brightness, which is reduced proportionally with a reduction in power level without any visible flickering or pulsing, has proved to be more difficult to achieve.
To get an even glow at less than 100% power levels, some cook tops employ a control system based on an alternating current switch in the form of a phased triac. However, this type of control system has a substantial drawback in that it requires a large heat sink as well as large inductors in series in order to work effectively. These systems also inherently create undesirably high dl/dt and dV/dt characteristics in the control signal and tend to be too expensive to use on all but top of the line appliances.
The electronic control circuit of the present invention provides a significant improvement over traditional triac based control systems of this type with full half cycle triac control rather than a phased triac approach. Power levels are created by turning the element ON for one or more full half cycles and then OFF for a whole number of half cycles in order to create a duty cycle approaching the desired power level. This provides adjustable heating element control and eliminates the high voltage and current rise rates implicit with phased triac control. The large heat sinks and series inductors needed for phased triac control are also reduced in terms of both size and cost. These and other features and advantages of the present invention will become apparent upon review of the following description taken in conjunction with the accompanying drawings.